Seismic sources, including vibratory sources, can be utilized in geophysical exploration in water covered areas of the earth. Signals can be generated by those sources in the water so that they travel downwardly into the Earth's seabed and are reflected from geological formations in the Earth and are detected by signal detectors such as hydrophones, geophones, accelerometers, pressure sensors, and such. In marine applications, the signals are generated in the body of water and travel into the Earth, while the signal detectors are in the body of water and can be near the seabed surface or the water surface.
Acoustic sources can be employed in marine operations that are impulsive (such as air guns). These are generally intended to generate as much energy as possible during as short a time span as possible. The frequency content of such sources is controllable only to a small degree, and different sources are selected for the generation of different frequency ranges for different surveying needs. The lack of frequency control can be relevant because, in connection with creating a large amount of energy, high frequency portions of the signal are normally produced. An issue with this is that impulsive sources can to a degree disrupt mammalian activities and therefore use of lower frequency sources that exclude high frequency components can be desirable. Additionally, it has been found that in various applications lower frequency sources are desirable for various surveys. Further, outside the lower frequency ranges, it is still desirable to have added frequency control and variation without requiring interchange of different sources.
One way of addressing the air gun limitations with respect to frequency has been with a marine vibrator. A marine vibrator produces a desired displacement of water at frequency. One way to drive a marine vibrator is with a hydraulic motor. However, there are some drawbacks with hydraulically driven vibrators, such as potential leakage of hydraulic fluid (oil) into the ocean. Also, hydraulically driven vibrators are inherently be non-linear and thus tend to produce an undesired harmonic.
An example of a hydraulically driven vibrator can be found in U.S. Pat. No. 4,153,135 that was filed on Sep. 30, 1977, which is incorporated herein by reference.
Another way of driving a marine vibrator is with a linear electromagnetic motor. The present disclosure relates to a number of embodiments of a linear electromagnetically driven marine vibrator having combinations of features that improve the design and performance of marine vibrators.